Low cement refractory

ABSTRACT

A low cement castable refractory comprising 85 to 96% by weight of a calcined ultrafine bauxite aggregate characterized by a particle size of less than 10 um, 2 to 10% by weight of a suitable cement material, 0 to 8% of weight of a thixotropic agent and 0 to 2% by weight of suitable deflocculants and plasticizers, the cement material comprising calcium aluminate cement having an alumina content of 40 to 85% and the thixotropic agents being selected from those containing microfine silica, and/or reactive alumina, the inorganic deflocculants and plasticizers being selected from alkali phosphates, lignosulphonates or naphthalene sulphonates. A method of making a low cement castable refractory and installing a low cement castable refractory are also described.

FIELD OF THE INVENTION

This invention relates to a low cement castable refractory and to aprocess for making the same.

BACKGROUND OF THE INVENTION

A number of low cement castable refractories have been developed for usewithin the aluminum industry. These castable refractories consist of ablend of calcined aggregate, cements and fillers which are mixed invarious proportions to suit the specific end applications. The calcinedaggregate is selected from a range of materials, including tabularalumina, bauxites, kaolins and other clays, and normally contributes upto 95 wt % of the final refractory mixture.

Characteristics and properties of low cement castables compared withconventional castable refractory materials include:

(i) A low moisture requirement during mixing. This results in a firedrefractory of higher density and lower porosity.

(ii) Higher cured and fired strengths, particularly in the temperaturerange of 600° to 1100° C.

(iii) Greater chemical and erosion resistance against molten metals andslags.

(iv) Higher refractories and greater thermal shock resistance.

Even though superior to conventional castables, a major disadvantage ofthe low cement castables currently available is their susceptibility tochemical attack and erosion. This necessitates a relatively frequentfurnace or crucible shutdown/replacement program and hence reducedproductivity. Where used as a barrier layer in reduction cells, agradual breakdown of the refractory over time leads to exposure of theunderlying insulation layers to the aggressive cell environment.Ultimately this will result in cell failure and shutdown, but prior tothis a gradual increase in cell voltage and hence a resulting loss inenergy efficiency.

A further disadvantage of low cement castables presently available istheir high cost compared with conventional castable refractories. Thisis largely an artifact of the aggregate material; premium grade calcinedminerals have been used exclusively for these applications.

SUMMARY OF THE INVENTION AND OBJECTS

There is clearly a need for a low cost, chemically resistant low cementcastable refractory for use in the aluminium industry and it is anobject of the present invention to provide such a castable refractoryand a method of providing same.

The invention provides a low cement castable refractory comprising 85 to96% by weight of a calcined ultrafine bauxite aggregate in which theultrafine bauxite forming the aggregate is characterised by agglomeratedparticles of a size of less than 6 um, 2 to 10% by weight of a suitablecement material, 0 to 8% by weight of thixotropic agent and 0 to 2% byweight of suitable deflocculants and plasticizers.

The calcined ultrafine bauxite aggregate is preferably characterised bya particle size of less than 1 um, such as the naturally occurring"ultrafine" bauxite which occurs at specific bauxite deposits, forexample, at the Weipa deposit in Northern Australia. Such ultrafinebauxite is usually recovered from settling and drying ponds in the formof large agglomerated lumps. The lumps are crushed to produce aggregateparticles in a range of sizes which are then calcined at hightemperatures to produce the calcined ultrafine bauxite aggregate. Thecalcined aggregate is screened and ground to produce a controlleddistribution of sized fractions, the following example of which has beenfound to provide suitable results in the low cement castable describedin this invention:

3 to 6 mm--10 to 40%

1 to 3 mm--5 to 30%

0.1 to 1 mm--5 to 30%

<0.1 mm--10 to 40%

The calcined ultrafine bauxite aggregate differs from conventionalcalcined refractory grade bauxite aggregates in that it is produced bycalcining agglomerated ultrafine bauxite particles. Conventionalcalcined refractory grade bauxite aggregates are produced by crushingand calcining bauxite ores recovered from the ground in an as-minedform.

The advantage of the calcined ultrafine bauxite aggregate lies in itshigh microstructural uniformity and low chemical reactivity. Suchuniformity cannot be achieved in conventional calcined refractory gradebauxite aggregates because of the inherent inhomogeneities found in thebauxite ores.

Of course acceptable results may be achieved through the communicationand re-agglomeration of refractory grade bauxite ores and although thiswould be a more expensive processing option the resulting castablerefractory would nevertheless still offer functional advantages over thecurrently known low cement castables.

The cement material may comprise calcium aluminate cement, whichpreferably has an alumina content of 40 to 85%.

The thixotropic agents preferably contain microfine silica and/orreactive alumina. The deflocculants and plasticizers preferably containalkali phosphates, lignosulphonates and napthalene sulphonates.

The new type of refractory has many uses, but is particularly suited inareas where resistance to molten aluminium and/or cryolite (Na₃ AlF₆) isrequired, for example, as a refractory for lining aluminium holdingfurnaces or for the construction of Hall Heroult reduction cells. In thelatter case, the new refractory is intended to serve as a barrier orprotection layer between the carbonaceious cathode and the underlyinginsulation.

The refractory material according to the invention has significantadvantages over conventional materials presently used for theseapplications; particularly its extreme chemical resistance to theaggressive molten aluminium and cryolite environment. Part of thischemical resistance can be attributed to the selection andcharacteristics of the refractory aggregate; in our invention a calcinedultrafine bauxitic based material.

The invention further provides a method of making a low cement castablerefractory comprising mixing together 85 to 96% by weight of a calcinedultrafine bauxite aggregate in which the ultrafine bauxite forming theaggregate comprises agglomerated particles having a size less than 6 um,2 to 10% by weight of a suitable cement, 0 to 8% by weight of athixotropic agent, and 0 to 2% by weight of suitable deflocculants andplasticizers.

The invention also provides a method of installing a low cementrefractory comprising mixing together 85 to 96% by weight of a calcinedultrafine bauxite aggregate comprising agglomerated particles having asize of less than 6 um, 2 to 10% by weight of a suitable cement, 0 to 8%by weight of thixotropic agents and 0 to 2% by weight of suitabledeflocculants and plasticizers with 2 to 10% by weight of water,followed by treatment to remove entrapped air, curing and heating to therefractory operating temperature.

In a preferred form of the invention, the ultrafine bauxite ispreferably a naturally occurring "ultrafine" bauxite which is derivedfrom the beneficiation of bauxite ore. The ultrafine bauxite material ispreferably agglomerated, solar dried to a moisture content of the orderof 0 to 10%, recovered, crushed and screened into acicular chips of thedesired granulometry and calcined at a temperature above 1350° C. Thecalcined ultrafine bauxite aggregate is preferably sized and ground togive a distribution in the range of:

3 to 6 mm--10 to 40%

1 to 3 mm--5 to 30%

0.1 to 1 mm--5 to 30%

<0.1 mm--10 to 40%

DESCRIPTION OF THE PREFERRED EMBODIMENT

One technique for producing a low cement castable refractory embodyingthe invention involves the following steps:

(i) Beneficiation of the bauxite ore to recover an agglomeratedultrafine fraction.

(ii) Solar drying of the ultrafine bauxite material to a desiredmoisture content (c.a. 0 to 10%).

(iii) Mining of the solar dried ultrafine material. The ultrafine formlumps of agglomerated cake.

(iv) Crushing and screening of the agglomerated ultrafine material, intoacicular chips of a desired granulometry (nominally less than 10 mm).

(v) Calcination of the ultrafine chips, at temperatures above 1350° C.The optimum calcination temperature will depend on the chemicalcomposition of the ultrafine bauxite.

(vi) Dry mixing of the calcined aggregate and other constitutents of therefractory to a desired composition.

The resulting castable mixture is then mixed with water, cured and firedto produce the final refractory material.

The first step in the preparation of the castable as described aboveinvolves the preparation of the calcined ultrafine bauxite aggregatematerial. Where naturally occurring, the ultrafine material must beseparated from the remainder of the deposit through beneficiation. Thisis best achieved through a water washing and screening process. Theultrafine material would then be pumped to a tailings dam for settlingand solar drying. As an optional stage, the ultrafine slurry may firstbe classified to ensure that all particles greater than 10 microns indiameter are removed.

During the solar drying process the ultrafines form a cake containingaround 0 to 10% moisture. This cake is then mined and crushed to anoptimum aggregate size distribution. The aggregate is then calcined attemperatures above 1350° C. Suitable calcination devices include, butare not restricted to, rotary kilns, fluidized beds and gas suspensioncalciners.

The refractory castable is produced in the following manner. Refractoryultrafine bauxite aggregate is sized to give a distribution in thefollowing range:

3 to 6 mm--10 to 40%

1 to 3 mm--5 to 30%

0.1 to 1 mm--5 to 30%

<0.1 mm--10 to 40%

85 to 96% by weight of this aggregate is dry mixed with 2 to 10% byweight of calcium aluminate cement, 0 to 8% by weight of a thixoptropicagent and 0 to 2% by weight of a suitable deflocculant and plasticizer.The calcium aluminate cement can be any commercially available productwith an alumina content ranging from 40 to 85%. The thixotropic agentmay be any commercially available product or products containingmicrofine silica and/or reactive alumina. The deflocculant andplasticizer may be any commercially available alkali phosphate orpolyphosphate, lignosulphate or napthalene sulphonate.

The calcium aluminate cement and chemical bonding and thixotropic agentare pre-mixed in a suitable powder mixing unit, for example in a V-typemixer. Mixing is carried out until a homogenous powder is obtained. Thepre-mixed components are then mixed with the aggregate fractions in atumble or ribbon type mixer. The dry mix is stored in air-tight bagsuntil ready for casting.

The castable is installed by mixing the dry castable with about 2 to 10%by weight of clean water and a high shear refractory concrete mixer isused to achieve mixing. Mixing is carried out until all particles areuniformly wetted. The castable is placed in position and vibrated withinternal poker vibrations or external surface vibrations. A vibrationfrequency of 3000-18000 vibrations per minute is recommended.

The castable composition is air cured for 24 hours before heating. Theheating rate must be slow enough to allow moisture to escape withoutmechanical damage. Heating or firing is continued until the operatingtemperature has been reached.

SPECIFIC EXAMPLE

Calcined ultrafine bauxite aggregate was produced with ultrafine bauxiteselectively mined from the Weipa beneficiation plant tailings dam. Thedried lumps were crushed in a jaw crusher and calcined in an oil firedrotary kiln at a temperature of 1400° to 1450° C. The fine fraction(-250 microns) was produced by wet grinding the coarser aggregate (3 to5 mm) in a ceramic lined ball mill with ceramic grinding balls.

The chemical compositions of the calcined ultrafine bauxite aggregate isgiven below:

    ______________________________________                                        Species     Analysis (wt %)                                                   ______________________________________                                        Al.sub.2 O.sub.3                                                                          72.6                                                              SiO.sub.2   13.8                                                              Fe.sub.2 O.sub.3                                                                          8.95                                                              TiO.sub.2   3.94                                                              Na.sub.2 O  0.06                                                              K.sub.2 O   0.03                                                              ______________________________________                                    

The calcined ultrafine bauxite aggregate was sized to the followingdistribution for the castable composition:

3.2 to 5.6 mm--32%

1.0 to 3.2 mm--23%

0.1 to 1.0 mm--21%

<0.1 mm--24%

The sized ultrafine aggregate was then blended with other constitutuentsin a laboratory V-mixer, in the following proportions:

    ______________________________________                                        Species                 Weight                                                ______________________________________                                        Calcined ultrafine bauxite aggregate                                                                  90.4                                                  Calcium aluminate cement (70% Al.sub.2 O.sub.3)                                                       4.6                                                   Microfine silica        3.75                                                  Reactive alumina        1.0                                                   Sodium polyphosphate    0.25                                                  ______________________________________                                    

The dry castable mix was then placed in a Hobart mixer and 5% by weightof water was added. Mixing was carried out until a ball in hand testindicated the material was ready for casting. The wet mix was cast in 75mm cubic blocks using a vibrating table operating at 11000 vibrationsper minute. The blocks were left in the moulds at room temperature for24 hours. They were dried at 100° C. for 24 hours and then heated to400° C. at 50° C./hour. They were held at 400° C. for 3 hours and firedto 1000° C. at 100° C./hour. They were held at 1000° C. for 2 hours andcooled down in the furnance.

The blocks were tested for density, porosity and cold crushing strengthaccording to ASTM and British Standards. The results are given below:

    ______________________________________                                                         Experimental Result                                          ______________________________________                                        Density (gcm.sup.-3)                                                                             2.8                                                        Porosity (%)       15                                                         Cold Crushing Strength (MPa)                                                                     85                                                         ______________________________________                                    

In order to determine the chemical resistance of the cast samples, 28 mmdiameter holes were drilled in the fired blocks to a depth of 35 mm.Molten aluminium and cryolite cup tests were carried out with 7075aluminium alloy and crylolite bath of the following composition:

NaF--53.9%

AlF₃ --29.4

CaF₂ --9.8

Al₂ O₃ --4.9

Al powder--2.0

The cryolite cup tests were carried out at 1000° C. for 24 hours and thealuminium cup tests at 1000° C. for 72 hours. After cooling, the sampleswere sectioned and the extent of attack on the refractory observed.

Comparative tests on a commercially available low cement castablerefractory with a similar alumina content showed that the extent ofattack by cryolite is far less severe in the case of with the castablecomposition according to the present invention. In the case of threecommercial low cement refractories the observed corroded areas were 3.1cm², 3.6 cm² and 2.5 cm², whereas the corroded area in a refractoryaccording to the invention was 0.9 cm², representing a significantimprovement.

In the case of the aluminium cup tests, the commercially available lowcement refractories were characterised by large black/grey areas ofcorrundum indicating attack and penetration by the molten aluminium ofthe refractory body. The refractory embodying the invention exhibited nowetting by the aluminium and no reaction, again indicating a significantimprovement.

The contents of the provisional specification accompanying AustralianPatent Application No. PJ 0780 is incorporated here by cross reference.

We claim:
 1. A low cement castable refractory comprising 85 to 96% byweight of a calcined ultrafine bauxite aggregate in which the ultrafinebauxite forming the aggregate is characterised by agglomerated particlesof a size of less than 6 um, 2 to 10% by weight of a suitable cementmaterial, 0 to 8% by weight of thixotropic agent and 0 to 2% by weightof suitable deflocculants and plasticizers.
 2. The refractory of claim1, wherein said calcined ultrafine bauxite aggregate sized and groundsubstantially according to the following distribution:3 to 6 mm--10 to40% 1 to 3 mm--5 to 30% 0.1 to 1 mm--5 to 30% <0.1 mm--10 to 40%
 3. Therefractory of claim 1, wherein said cement material is calcium aluminatecement which has an alumina content of about 40 to 85%.
 4. Therefractory of claim 1, wherein said thixotropic agents contain materialsselected from microfine silica, and/or reactive alumina, and saiddeflocculants and plasticizers are selected from alkali phosphates,lignosulphonates or naphthalene sulphonates.
 5. A method of making a lowcement castable refractory comprising mixing together 85 to 96% byweight of a calcined ultrafine bauxite aggregate in which the ultrafinebauxite forming the aggregate comprises agglomerated particles having asize less than 6 um, 2 to 10% by weight of a suitable cement, 0 to 8% byweight of a thixotropic agent, and 0 to 2% by weight of suitabledeflocculants and plasticizers.
 6. A method of installing the low cementcastable refractory of claim 5 by mixing the dry low cement castablerefractory mix with 2 to 10% by weight of water followed by treatment toremove entrapped air, curing and heating to the refractory operatingtemperature.
 7. The method of claim 5, wherein said ultrafine bauxiteparticles are agglomerated, dried to a moisture content of the order of2 to 10%, recovered, crushed and screened into acicular chips of thedesired granulometry and calcined at a temperature above 1350° C.
 8. Themethod of claim 5, wherein said ultrafine bauxite aggregate is sizedsubstantially according to the following distribution:3 to 6 mm--10 to40% 1 to 3 mm--5 to 30% 0.1 to 1 mm--5 to 30% <0.1 mm--10 to 40%